Pad Window Insert

ABSTRACT

A polishing pad includes a polishing layer having a polishing surface, an adhesive layer on a side of the polishing layer opposite the polishing layer, and a solid light-transmitting window extending through and molded to the polishing layer. The solid light-transmitting window has an upper portion with a first lateral dimension and a lower portion with a second lateral dimension that is smaller than the first lateral dimension. A top surface of the solid light-transmitting window coplanar with the polishing surface and a bottom surface of the solid light-transmitting window coplanar with a lower surface of the adhesive layer.

TECHNICAL FIELD

A polishing pad with a window, a system containing such a polishing pad,and a process for making and using such a polishing pad are described.

BACKGROUND

In the process of fabricating modern semiconductor integrated circuits(IC), it is often necessary to planarize the outer surface of thesubstrate. For example, planarization may be needed to polish away aconductive filler layer until the top surface of an underlying layer isexposed, leaving the conductive material between the raised pattern ofthe insulative layer to form vias, plugs and lines that provideconductive paths between thin film circuits on the substrate. Inaddition, planarization may be needed to flatten and thin an oxide layerto provide a flat surface suitable for photolithography.

One method for achieving semiconductor substrate planarization ortopography removal is chemical mechanical polishing (CMP). Aconventional chemical mechanical polishing (CMP) process involvespressing a substrate against a rotating polishing pad in the presence ofan abrasive slurry.

In general, there is a need to detect when the desired surface planarityor layer thickness has been reached or when an underlying layer has beenexposed in order to determine whether to stop polishing. Severaltechniques have been developed for the in-situ detection of endpointsduring the CMP process. For example, an optical monitoring system forin-situ measuring of uniformity of a layer on a substrate duringpolishing of the layer has been employed. The optical monitoring systemcan include a light source that directs a light beam toward thesubstrate during polishing, a detector that measures light reflectedfrom the substrate, and a computer that analyzes a signal from thedetector and calculates whether the endpoint has been detected. In someCMP systems, the light beam is directed toward the substrate through awindow in the polishing pad.

SUMMARY

A window can be attached to the underside of a polishing pad such that aportion of the window rests in a recess in a platen. This can permit alarge surface area contact between the window and the pad so as toincrease bonding strength between the window and the polishing pad.

In one aspect, a polishing apparatus includes a platen having a planarupper surface, a recess formed in the upper surface, the recess having abottom surface, and a passage connected to the lower surface of therecess, as well as a polishing pad comprising a polishing layer, apolishing surface, and underside and an aperture therethrough, theaperture having a smaller lateral dimension than the recess, theaperture aligned with the passage. A solid light-transmitting window hasa first portion positioned at least partially in the aperture in thepolishing pad and a second portion positioned at least partially in therecess in the platen, the second portion having a larger lateraldimension than the first portion and extending below the polishinglayer, the second portion of the window adhesively attached to anunderside of the polishing pad.

Implementations can include one or more of the following features. Thefirst portion of the window can plug the aperture in the polishing pad.A top surface of the first portion of the window can be coplanar withthe upper surface of the platen. The bottom surface of the recess can beparallel with the upper surface of the platen. A lower surface of thesecond portion of window can contact the lower surface of the recess.The lower surface of the second portion of window may not be adhered tothe lower surface of the recess. The polishing apparatus can alsoinclude an adhesive layer spanning the polishing layer. The adhesivelayer can include a double-sided adhesive tape. The adhesive layer canabut the polishing layer. The underside of the polishing pad can beadhesively attached to the upper surface of the platen by the adhesivelayer. A top surface of the second portion of the window can beadhesively attached to underside of the polishing pad by the adhesivelayer. A top surface of the second portion of the window can beadhesively attached to underside of the polishing pad. The polishing padcan include the polishing layer. The polishing pad can include thepolishing layer and a lower layer that is less compressible than thepolishing layer. The second portion can have a lateral dimension betweentwo and ten times larger, e.g., about eight times larger, than the firstportion. The second portion of the window can laterally fill the recessin the platen. The polishing pad can have a thickness less than 1 mm.The polishing apparatus can also include an optical fiber in the passageand positioned to direct or receive light through the first portion ofthe window. The optical fiber can be wider than the first portion of thewindow. The sides of the recess can be sloped and sides of the secondportion of the window can be sloped.

In another aspect, method of assembling a window for a polishingapparatus includes forming an aperture through a polishing pad, thepolishing pad comprising a polishing layer having a polishing surfaceand underside, forming a solid light-transmitting window having a firstportion and a second portion having a larger lateral dimension than thefirst portion, inserting the first portion of the window into theaperture of the pad, adhering a top surface of the second portion of thewindow to the underside of the polishing pad, and positioning thepolishing pad and window on a platen such that the second portion of thewindow fits into a recess in a planar upper surface of the platen andthe underside of the polishing pad is adhered to the planar uppersurface of the platen.

Implementations can include one or more of the following features. Alayer of adhesive can be formed on the bottom of the polishing layer anda liner covers the adhesive, a portion of the liner can be removedaround the aperture, and the top surface of the second portion of thewindow can contact the adhesive in the removed portion of the liner.

Implementations may include the following potential advantages. A strongbond can be formed between the window and a thin polishing pad, reducingthe likelihood of slurry leakage and reducing the likelihood of thewindow being pulled from the pad due to shear force from the substratebeing polished. In addition, the polishing pad can improvewafer-to-wafer uniformity of spectrum reflected from the substrate,particularly at short wavelengths.

The details of one or more embodiments are set forth in the accompanyingdrawings and the description below. Other aspects, features andadvantages will be apparent from the description and drawings, and fromthe claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of a CMP apparatus containing apolishing pad.

FIG. 2 is a top view of an embodiment of a polishing pad with a window.

FIG. 3A is a cross-sectional view of the polishing pad of FIG. 2installed on a platen.

FIG. 3B is a cross-sectional view of the polishing pad of FIG. 2.

FIGS. 4-7 illustrate a method of forming a polishing pad.

FIG. 8 is a cross-sectional view of another implementation the polishingpad installed on a platen.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

A window can be attached to the underside of a polishing pad such that aportion of the window rests in a recess in a platen. This can permit alarge surface area contact between the window and the pad so as toincrease bonding strength between the window and the polishing pad.

As shown in FIG. 1, the CMP apparatus 10 includes a polishing head 12for holding a semiconductor substrate 14 against a polishing pad 18 on aplaten 16. The CMP apparatus may be constructed as described in U.S.Pat. No. 5,738,574, the entire disclosure of which is incorporatedherein by reference.

The substrate can be, for example, a product substrate (e.g., whichincludes multiple memory or processor dies), a test substrate, a baresubstrate, and a gating substrate. The substrate can be at variousstages of integrated circuit fabrication, e.g., the substrate can be abare wafer, or it can include one or more deposited and/or patternedlayers. The term substrate can include circular disks and rectangularsheets.

The effective portion of the polishing pad 18 can include a polishinglayer 20 with a polishing surface 24 to contact the substrate and abottom surface 22 secured to the platen 16 by an adhesive layer 28,e.g., an adhesive tape. The adhesive 28 can be a pressure-sensitiveadhesive. Other than the adhesive tape and any liner, the polishing padcan be, e.g., consist of, a single-layer pad, with the polishing layer20 formed of a thin durable material suitable for a chemical mechanicalpolishing process. Thus, the layers of the polishing pad can consist ofthe single-layer polishing layer 20 and the adhesive layer 28 (andoptionally a liner, which would be removed when the pad is installed onthe polishing platen).

The polishing layer 20 can be, e.g., consist of, a foamed polyurethane,with at least some open pores on the polishing surface 24. The adhesivelayer 28 can be a double-sided adhesive tape, e.g., a thin layer ofpolyethylene terephthalate (PET), e.g., Mylar®, with adhesive, e.g.,pressure-sensitive adhesive, on both sides. Such a polishing pad isavailable under the trade name H7000HN from Fujibo in Tokyo, Japan.

Referring to FIG. 2, in some implementations the polishing pad 18 has aradius R of 15.0 (381.00 mm) to 15.5 inches (393.70 mm), with acorresponding diameter of 30 to 31 inches. In some implementations, thepolishing pad 18 can have a radius of 21.0 (533.4 mm) to 21.5 inches(546.1 mm), with corresponding diameter of 42 to 43 inches.

Referring to FIG. 3A, in some implementations, grooves 26 can be formedin the polishing surface 24. The grooves can be in a “waffle” pattern,e.g., a cross-hatched pattern of perpendicular grooves with sloped sidewalls that divide the polishing surface into rectangular, e.g., square,areas.

Returning to FIG. 1, typically the polishing pad material is wetted withthe chemical polishing liquid 30, which can include abrasive particles.For example, the slurry can include KOH (potassium hydroxide) andfumed-silica particles. However, some polishing processes are“abrasive-free”.

The polishing head 12 applies pressure to the substrate 14 against thepolishing pad 18 as the platen rotates about its central axis. Inaddition, the polishing head 12 is usually rotated about its centralaxis, and translated across the surface of the platen 16 via a driveshaft or translation arm 32. The pressure and relative motion betweenthe substrate and the polishing surface, in conjunction with thepolishing solution, result in polishing of the substrate.

An optical aperture 34 is formed in the top surface of the platen 16. Anoptical monitoring system, including a light source 36, such as a laser,and a detector 38, such as a photodetector, can be located below the topsurface of the platen 16. For example, the optical monitoring system canbe located in a chamber inside the platen 16 that is in opticalcommunication with the optical aperture 34, and can rotate with theplaten. One or more optical fibers 50 can carry light from the lightsource 36 to the substrate, and from the substrate to the detector 38.For example, the optical fiber 50 can be a bifurcated optical fiber,with a trunk 52 in proximity, e.g., abutting, the window 40 in thepolishing pad, a first leg 54 connected to the light source 36, and asecond leg 56 connected to the detector 38.

The optical aperture 34 can be filled with a transparent solid piece,such as a quartz block (in which case the fiber would not abut thewindow 40 but could abut the solid piece in the optical aperture), or itcan be an empty hole. In one implementation, the optical monitoringsystem and optical aperture are formed as part of a module that fitsinto a corresponding recess in the platen. Alternatively, the opticalmonitoring system could be a stationary system located below the platen,and the optical aperture could extend through the platen. The lightsource 36 can employ a wavelength anywhere from the far infrared toultraviolet, such as red light, although a broadband spectrum, e.g.,white light, can also be used, and the detector 38 can be aspectrometer.

A solid window 40 is formed in the overlying polishing pad 18 andaligned with the optical aperture 34 in the platen. The window 40 andaperture 34 can be positioned such that they have a view of thesubstrate 14 held by the polishing head 12 during at least a portion ofthe platen's rotation, regardless of the translational position of thehead 12.

The light source 36 projects a light beam through the aperture 34 andthe window 40 to impinge the surface of the overlying substrate 14 atleast during a time when the window 40 is adjacent the substrate 14.Light reflected from the substrate forms a resultant beam that isdetected by the detector 38. The light source and the detector arecoupled to an unillustrated computer that receives the measured lightintensity from the detector and uses it to determine the polishingendpoint, e.g., by detecting a sudden change in the reflectivity of thesubstrate that indicates the exposure of a new layer, by calculating thethickness removed from of the outer layer (such as a transparent oxidelayer) using interferometric principles, by monitoring the spectrum ofthe reflected light and detecting a target spectrum, by matching asequence of measured spectra to reference spectra from a library anddetermining where a linear function fit to index values of the referencespectrum reaches a target value, or by otherwise monitoring the signalfor predetermined endpoint criteria.

One problem with placement of a normal large rectangular window (e.g., a2.25 by 0.75 inch window) into a very thin polishing layer isdelamination during polishing. In particular, the lateral frictionalforce from the substrate during polishing can be greater than theadhesive force of the molding of the window to the sidewall of the pad.

Returning to FIG. 2, the window 40 can be small, e.g., less than about 3mm in diameter, e.g., so as to reduce the frictional force applied bythe substrate during polishing. For example, the upper portion of thewindow 40 can be a circular area about 3 mm wide centered a distance Dof about 7.5 inches (190.50 mm) from the center of a 30 to 31 inchdiameter polishing pad 18, or centered a distance D of about 9 to 11inches from the center of a 42 to 43 inch diameter polishing pad 18.

The window 40 can have an approximately circular shape (other shapes arepossible, such as rectangular). If the window is elongated, its longerdimension can be substantially parallel to the radius of the polishingpad that passes through the center of the window. The window 40 can havea ragged perimeter 42, e.g., the perimeter can be longer than aperimeter of a similarly shaped circle or rectangle, e.g., a zig-zag orother meandering pattern (in a top view). This increases the surfacearea for contact of the window to the sidewall of the polishing pad, andcan thereby improve adhesion of the window to the polishing pad.

Referring to FIG. 3A, the window 40 includes an upper portion 40 a and alower portion 40 b. The window 40, including the upper portion 40 a andlower portion 40 b, can be a unitary single-piece body of homogeneousmaterial. The upper portion 40 a is vertically aligned with the lowerportion 40 b but is laterally smaller (i.e., in one or both directionsparallel to the polishing surface) than the lower portion 40 b. Thus, aportion of the polishing layer 20 projects above the lower portion 40 bso that the rim of the lower portion 40 b that projects beyond the upperportion 40 a forms a ledge 49. The lower portion 40 b can projectlaterally beyond the upper portion 40 a on all sides of the window 40,or optionally the lower portion 40 b can project laterally beyond theupper portion 40 a on two opposing sides of the window 40 but be alignedalong other sides of the window 40. The upper surface of the lowerportion 40 b that projects beyond the upper portion 40 a can be asubstantially planar surface. The upper portion 40 a can be located inthe center of, e.g., be concentric with, the lower portion 40 b. Thelower portion 40 b can have a lateral dimension 2 to 10 times, e.g.,about 8 times, as large as the lateral dimension of the upper portion 40a. For example, if the window 40 is circular, the upper portion 40 a canhave a diameter of 3 mm, and the lower portion 40 b can have a diameterof 25 mm.

The upper portion 40 a can be about the same thickness as the lowerportion 40 b. Alternatively, the upper portion 40 a can be thicker than,or be thinner than, the lower portion 40 b.

The upper portion 40 a of the window 40 can project into an aperture inthe adhesive layer 28. The edge of the adhesive layer 28, e.g., adhesivetape, can abut the sides of the upper portion 40 a of the window 40. Thelower portion 40 b of the window can project into a recess 78 in the topsurface 76 of the platen 16.

The upper portion 40 a of the window is as thick as the combination ofthe polishing layer 20 and the adhesive layer 28. A top surface 44 ofthe upper portion 40 a of the window 40 is coplanar with the polishingsurface 24. A bottom of the upper portion 40 a of the window 40 can becoplanar with a bottom surface of the adhesive layer 28.

The upper surface of the lower portion 40 b is secured to the undersideof the polishing layer 20 by a portion of the adhesive layer 28.Optionally the perimeter of the upper portion 40 a of the window 40 canbe secured to the inner sidewall edges 48 of the polishing layer 20,e.g., by additional adhesive.

The increased surface area of connection between the window 40 and thepolishing layer 20 provided by the connection on the ledge 49 canprovide a strong bond, reducing the likelihood of slurry leakage andreducing the likelihood of the window 40 being pulled from the polishingpad 18 due to shear force from the substrate being polished. The trunk52 of the optical fiber abuts or nearly abuts the lower portion 40 b. Insome implementations, the trunk 52 may be wider than the upper portion40 a of the window 40.

The bottom surface of the lower portion 40 b of the window can abut,e.g., rests on, without being adhesively or otherwise secured to thebottom of the recess 78 in the upper surface 76 of the platen 16. Insome implementations, the lower portion 40 b of the window fills therecess 78.

Referring to FIG. 3B, before installation on the platen 16, thepolishing pad 18 can also include a liner 70 that spans the adhesivelayer 28 on the bottom surface 22 of the polishing pad 18, except in theregion covered by the adhesive lower portion 40 b of the window 40. Theliner 70 can be a thin flexible material, e.g., paper, with a releasecoating so that it can be peeled away from the adhesive 28. In someimplementations, the liner can be an incompressible and generallyfluid-impermeable layer, for example, polyethylene terephthalate (PET),e.g., Mylar®. In use, the liner 70 is manually peeled from the adhesivelayer 28, and the polishing layer 20 is applied to the platen 16 withthe adhesive layer 28. The liner 70, however, does not span the window40, but is removed in and immediately around the region of the lowerportion 40 b of the window 40, e.g., in a region about 25 cm across, toform a hole 72 into which the lower portion 40 b of the window 40 fits.

The polishing pad 18 is very thin, e.g., less than 2 mm, e.g., less than1 mm. For example, the total thickness of the polishing layer 20,adhesive 28 and liner 70 can be about 0.8 or 0.9 mm. The polishing layer20 can be about 0.7 or 0.8 mm thick, with the adhesive 28 and the liner70 providing about another 0.1 mm. The grooves 26 can be about half thedepth of the polishing pad, e.g., roughly 0.5 mm.

To manufacture the polishing pad, initially the polishing layer 20 isformed and the bottom surface of the polishing layer 20 is covered withthe pressure sensitive adhesive 28 and a liner 70, as shown by FIG. 4.Grooves 26 can be formed in the polishing layer 20 as part of a padmolding process before attachment of the pressure sensitive adhesive 28,or cut into the polishing layer 20 after the pad is formed. The grooves26 can be formed before or after the liner 70 is attached.

Referring now to FIG. 5, in some implementations the window 40 may beformed by casting and curing a polymer in the shape of the window 40. Inone implementation, the polymer is a mixture of 2 parts Calthane A 2300and 3 parts Calthane B 2300 (available from Cal Polymers, Inc. of LongBeach, Calif.). The liquid polymer mixture can be degassed, e.g., for15-30 minutes, before being placed into the aperture. The polymer can becured at room temperature for about 24 hours, or a heat lamp or oven canbe used to decrease cure time. In some implementations, the polymer canbe poured into a mold and cured or otherwise solidified to form thewindow 40 in its final shape. In some implementations, the window 40 cancured in a large solid block, and then can be formed by machining thesolid block of polymer to form the final shape of the window 40.

In some implementations, a sidewall 84 of the lower portion 40 b may besubstantially perpendicular to the bottom surface 46 of the window 40.In some implementations, the sidewall 84 may be formed at an angle tothe bottom surface 46, as will be discussed further in the descriptionof FIG. 7.

A hole 82 is punched through the entire polishing pad 18, including thepolishing layer 20, the adhesive 28 and the liner 70. The hole 82 issized to accommodate the upper portion 40 a of the window 40. In someimplementations, the upper portion 40 a substantially plugs the hole 82of the polishing pad 18. The hole 82 can be punched from the top (i.e.,the side with the polishing surface) of the pad, e.g., by a machinepress. This permits the position of the hole 82 to be positioned andsized with a high degree of accuracy and repeatability.

A portion 72 of the liner 70 is peeled away or otherwise removed fromthe adhesive layer 28. The liner 70 need not be peeled of the polishingpad 18 entirely at this time. The portion 72 of the liner 70 that ispeeled away exposes a portion of a bottom surface 22 of the adhesivelayer 28 around the hole 82. The portion 72 that is peeled off can alsobe cut away, e.g., in a region sized to accommodate the ledge 49 of thebottom portion 40 b of the window 40, although this step can beperformed at a later time.

Referring to FIGS. 5 and 6, the window 40 is secured to the polishingpad 18 such that the upper portion 40 a extends into the hole 82, andthe upper surface of the bottom portion 40 b (e.g., the ledge 49)contacts the adhesive layer 28. In some embodiments, the upper portion40 a is sized to extend substantially through and substantially fill thehole 82 such that the upper surface 44 is coplanar with the polishingsurface 24 of the polishing layer 20 when the ledge 49 is adhered to theadhesive layer 28.

In addition to the liner 70, an optional window backing piece 74 can beplaced to span the window 40. For example, the window backing piece 74can be secured to a portion of the adhesive layer 28 immediately aroundthe window 40. The backing piece 74 can be the same thickness as theliner 70, or thinner than the liner 70. The backing piece 74 can bepolytetrafluoroethylene (PTFE), e.g., Teflon®, or another non-stickmaterial. The combined polishing pad 18 and window 40 can then bereadied for shipment to the customer, e.g., in a sealed plastic bag.

Referring now to FIG. 7, when the customer receives the combinedpolishing pad 18 and window 40, the customer can remove the liner 70(and window backing piece 74 if present) and then attach the polishingpad 18 on the platen 16 using the adhesive layer 28. The lower portion40 b of the window 40 is inserted into a recess 78 in the upper surface76 of the platen 16. In some methods, the liner 70 can be peeled awaypartially in an area around the window 40, the lower portion 40 b of thewindow 40 is inserted into the recess, and then the remainder of theliner is peeled away and the rest of the polishing pad is secured to theplaten 16.

The lower portion 40 b can be shaped and sized to substantially fill therecess 78, for example, the sidewall 84 of the lower portion 40 b cancontact substantially all of a sidewall 86 of the recess 78, and thebottom surface 46 of the window 40 substantially contacts a floor 88 ofthe recess 78 while the upper surface 76 of the platen 16 contacts theadhesive layer 28.

In some implementations, the floor 88 of the recess 78 may besubstantially parallel with the upper surface 76 of the platen 16. Insome implementations, the sidewall 84 of the lower portion 40 b isperpendicular to the bottom surface 46, and the sidewall 86 of therecess 78 is perpendicular to the polishing surface 75. Referring toFIG. 8, in some implementations, the sidewall 84 of the lower portion 40b may be formed at a non-perpendicular angle, e.g., between 20° and 80°,e.g., 45°, to the bottom surface 46, and the sidewall 86 of the recess78 may be formed at a substantially similar angle such that the sidewall84 and the sidewall 86 substantially contact each other when the lowerportion 40 b is inserted into the recess 78. For example, the sidewall84 may slope inward from the ledge 49 to the bottom surface 46, suchthat the lower portion 40 b forms a conic section. Similarly, thesidewall 86 may be formed to mate with the conic section. As such, thesloped sidewall 84 may cause the window 40 and the polishing pad 18 toexhibit a self-centering property as the window 40 is inserted into therecess 78 within the sloped sidewall 86.

As such, the polishing pad 18 is adhered to the platen 16 by theadhesive layer 28, thereby retaining the window 40 within the recess 78in the platen 16. The window 40 can be vertically supported by the floor88 of the recess 78 and can be retained laterally by the sidewalls 86 ofthe recess 78. The window 40 can be adhered to the polishing pad by thecontact of the top surface of the ledge 49 to the same adhesive layerthat secures the underside of the polishing pad to the platen 16.

While certain embodiments have been described, the invention is not solimited. For example, although a window with a simple circular shape isdescribed, the window could be more complex, such as a rectangle, ovalor star. The top portion of the window can project past one or moresides of the bottom portion. It will be understood that various othermodifications may be made without departing from the spirit and scope ofthe invention. Accordingly, other embodiments are within the scope ofthe following claims.

1. A polishing apparatus, comprising: a platen having a planar uppersurface, a recess formed in the upper surface, the recess having abottom surface, and a passage connected to the lower surface of therecess; a polishing pad comprising a polishing layer a polishingsurface, and underside and an aperture therethrough, the aperture havinga smaller lateral dimension than the recess, the aperture aligned withthe passage; and a solid light-transmitting window having a firstportion positioned at least partially in the aperture in the polishingpad and a second portion positioned at least partially in the recess inthe platen, the second portion having a larger lateral dimension thanthe first portion and extending below the polishing layer, the secondportion of the window adhesively attached to an underside of thepolishing pad.
 2. The polishing apparatus of claim 1, wherein the firstportion of the window plugs the aperture in the polishing pad.
 3. Thepolishing apparatus of claim 2, wherein a top surface of the firstportion of the window is coplanar with the upper surface of the platen.4. The polishing apparatus of claim 1, wherein the bottom surface of therecess is parallel with the upper surface of the platen.
 5. Thepolishing apparatus of claim 1, wherein a lower surface of the secondportion of window contacts the lower surface of the recess.
 6. Thepolishing apparatus of claim 5, wherein the lower surface of the secondportion of window is not adhered to the lower surface of the recess. 7.The polishing apparatus of claim 1, further comprising an adhesive layerspanning the polishing layer.
 8. The polishing apparatus of claim 7,wherein the adhesive layer comprises a double-sided adhesive tape. 9.The polishing apparatus of claim 7, wherein the adhesive layer abuts thepolishing layer.
 10. The polishing apparatus of claim 7, wherein theunderside of the polishing pad is adhesively attached to the uppersurface of the platen by the adhesive layer.
 11. The polishing apparatusof claim 10, wherein a top surface of the second portion of the windowis adhesively attached to underside of the polishing pad by the adhesivelayer.
 12. The polishing apparatus of claim 7, wherein a top surface ofthe second portion of the window is adhesively attached to underside ofthe polishing pad.
 13. The polishing apparatus of claim 1, wherein thepolishing pad consists of the polishing layer.
 14. The polishingapparatus of claim 1, wherein the polishing pad consists of thepolishing layer and a lower layer that is less compressible than thepolishing layer.
 15. The polishing apparatus of claim 1, wherein thesecond portion has a lateral dimension between two and ten times largerthan the first portion.
 16. The polishing apparatus of claim 15, whereinthe second portion has a lateral dimension about 8 times larger than thefirst portion.
 17. The polishing apparatus of claim 1, wherein thesecond portion of the window laterally fills the recess in the platen.18. The polishing apparatus of claim 1, wherein the polishing pad has athickness less than 1 mm.
 19. The polishing apparatus of claim 1,further comprising an optical fiber in the passage and positioned todirect or receive light through the first portion of the window.
 20. Thepolishing apparatus of claim 19, wherein the optical fiber is wider thanthe first portion of the window.
 21. The polishing apparatus of claim 1,wherein sides of the recess are sloped and sides of the second portionof the window are sloped.
 22. A method of assembling a window for apolishing apparatus, comprising: forming an aperture through a polishingpad, the polishing pad comprising a polishing layer having a polishingsurface and underside; forming a solid light-transmitting window havinga first portion and a second portion having a larger lateral dimensionthan the first portion; inserting the first portion of the window intothe aperture of the pad; adhering a top surface of the second portion ofthe window to the underside of the polishing pad; and positioning thepolishing pad and window on a platen such that the second portion of thewindow fits into a recess in a planar upper surface of the platen andthe underside of the polishing pad is adhered to the planar uppersurface of the platen.
 23. The method of claim 22, wherein a layer ofadhesive is formed on the bottom of the polishing layer and a linercovers the adhesive, a portion of the liner is removed around theaperture, and the top surface of the second portion of the windowcontacts the adhesive in the removed portion of the liner.
 24. Themethod of claim 23, further comprising removing a remainder of the linerbefore positioning the polishing pad on the platen such that theadhesive adheres the underside of the polishing pad to the planar uppersurface of the platen.